Optical signal transmission network

Abstract: An optical signal transmission network comprises a main optical fibre (8) having a plurality of branch circuits (11,12), with each final branch circuit of a chain of branches ending in a terminal unit (15,16,17), and the network being associated with OTDR equipment () connected so as to transmit an output signal through the main fibre (8) and the branch circuits (11,12), wherein each said final branch circuit incorporates a filter (25,26,27) which is capable of reflecting back towards the OTDR equipment () at least one wavelength in the OTDR band, but not traffic wavelengths.

Description: This invention relates to optical signal transmission networks, as may be used, for example, in optical fibre telecommunications, and relates, in particular, to a fault location and diagnostic systems for use with such networks. Over the past decade, optical time domain reflectometry, hereinafter referred to as OTDR, has become an established tool for the location and evaluation of the features on an optical fibre route. For example, OTDR techniques are now commonly used to determine both the position and loss of splices and connectors and the position of line breads as well as providing a means for making single-ended, real-time system loss measurements. In use of an optical time domain reflectometer a short pulse of light is launched into an optical fibre under test and the backscattered signal is monitored as a function of the time of flight (or equivalently distance) along the fibre. The magnitude of the backscattered signal depends on the Rayleigh scattering, attenuation, inhomogeneities, splices, components and the optical power level in the fibre. Features in the trace of backscatter versus time may then be correlated with the presence of discontinuities and disruptions in the fibre route. Whilst OTDR techniques are extremely valuable as a diagnostic tool for the location of faults in simple point to point links there is now an increasing trend towards more complex passive optical networks. In such networks a number of branch circuits may emanate from a single optical fibre connected, for example to a telephone exchange, and the branch circuits may be further divided into further branch circuits, with each final branch circuit of a chain of branches ending in a respective terminal unit, for example at a customer's premises. Such an arrangement can provide a significant cost reduction by utilising the available bandwidth to share said single exchange fibre amongst a number of customers, thereby reducing the level of exchange equipment and fibre installation costs borne by each customer. It will, however, be apparent that when a signal from OTDR equipment is transmitted along said single fibre, and thence to the branch circuits, the signal which is returned at any instant, following the transmission of an output signal from the OTDR equipment, will be made up of light backscattered from many points in the network at the same distance from the equipment. This makes the location of a fault in the network difficult and time-consuming. An object of the invention is to provide a means of simplifying such fault location. European Patent Application No. EP 0 604 045 A2, which document falls within the terms of Article 54(3) EPC, describes an apparatus and a method for detecting and evaluating optical sub-circuits created by optical splitters in an optical circuit. One or more optical wavelengths are utilized for the transmission of data, whereas other optical wavelengths are utilized to perform optical time domain reflectometry measurements on each optical sub-circuit. Following an optical splitter, each optical path from the optical splitter has inserted into it an optical filter which allows the transmission of the optical wavelengths utilized for data transmission and one of the optical wavelengths utilized to perform the optical time domain reflectometry instrument transmits each of the optical wavelengths, utilized for testing, individually into the optical circuit and analyses the returned optical pulse. Since only one optical sub-circuit allows the passage of any given optical wavelength for testing, each optical sub-circuit can be fully analysed. European Patent Application No. EP 0 432 734 A2 discloses a fault distinguishing device for establishing whether fault exists in an optical fiber cable and in another optical system. The disclosed device is intermediate between the optical fiber cable and another optical system and optically coupled with the optical fiber cable and another optical system. The disclosed device is composed of at least one intrinsic optical fiber and at least one optical filter. The optical filter is internal to a corresponding intrinsic optical fiber and forms an angle of between 0 and 4.7 degrees inclusive with respect to a plane perpendicular to the longitudinal axis of the intrinsic optical fiber, such that the optical fiber is capable of reflecting a first light component and (λ t ) and of transmitting a second light component (λ o ). The first light component (λ o ) differ from one another in terms of wavelength. According to the present invention there is provided fault location apparatus for use in an optical signal transmission network of a type which includes a main optical fibre having a plurality of branch fibres, in which each final branch fibre of a chain of branches ends in a terminal unit, said fault location apparatus comprising: optical time domain reflectometry (OTDR) equipment connected to the optical transmission network so as to transmit an output signal through the main fibre and the branch fibres; and a filter incorporated in each final branch fibre which reflects towards the OTDR equipment a wavelength or wavelengths unique to that branch in the OTDR band, but not traffic wavelengths, and wherein said OTDR equipment is operated to make measurement on a "virtual" or "mirror" image of the path from the terminal unit and has means for distinguishing the signals returned from individual branch fibres. The filter may be disposed in the traffic line itself, in which case it should be substantially transmissive to traffic wavelengths, or it may be located in a spur line connected thereto, in which case it may either transmit or attenuate traffic wavelengths. Preferably, each filter is switchable between a first condition in which it transmits or attenuates wavelengths in the OTDR band, and a second condition in which it reflects wavelengths in the OTDR band, and means are provided for switching said filters individually into said second condition whilst maintaining the remaining filters in the first condition. It will be seen that in each of these embodiments the route to any final branch circuit having a fault can be unambiguously identified, by the signals returned to the OTDR equipment. Preferably in the second embodiment of the invention each said filter is located in or adjacent to the respective terminal unit. Such an arrangement has the advantage that a supply of power for actuating the switching of the filter will normally be readily available. Moreover it enables the filters to be easily interrogated and switched from the first to the second condition, avoiding the need for them to be constantly powered. The invention will be further explained by way of example with reference to the accompanying drawing, in which: Figure 1 represents in diagrammatic form a typical OTDR system, Figure 2 represents part of an optical fibre telecommunication network incorporating the invention, and Figure 3 represents a modification thereof. Referring first to Figure 1, the OTDR equipment illustrated comprises an electrical pulse generator 1, the output of which is fed to means 2 for producing a light signal in a selected waveband, and this is fed, in turn, into an optical fibre 3. When a signal is generated by the OTDR equipment, a signal will be returned along the fibre 3 made up of light backscattered from splices, disconformities and possible faults in the fibre or in customers' equipment, the returned signal being fed via a directional coupler or beam splitter 4 to a convertor 5 and detector 6 responsive to the returned signal, and display means for indicating the signal value, ie. after selected round trip delay times, and this accordingly enables the location of any fault in the fibre to be readily determined. Such an arrangement would operate quite satisfactorily where there is only a single output fibre. However, where the fibre 3 has connected to it a plurality of branch circuits, as, for example in the case of an optical telecommunication system serving a plurality of customers' premises, the returning signal at any instant will be made up of light backscattered from many individual points at the same distance from the OTDR equipment, making the extraction of useful information from the equipment extremely complex. Figure 2 illustrates how the invention avoids this difficulty, and represents, in simplified form, part of an optical fibre telecommunication network comprising a main fibre 8 connected to a telephone exchange (not shown) and feeding two branch circuits 11, 12. The branch circuit 12 feed, in turn, two further branch circuits 13, 14, and each of the branch circuits 11, 13, 14 terminate in respective customers' equipment designated schematically at 15, 16, 17. OTDR equipment, shown diagrammatically at 9 feeds output signals into the fibre 8 through suitable coupling means 10, the signals being transmitted from the fibre 8 into the various branch circuits. In accordance with the invention each of the customers' equipment incorporates a switchable filter 25, 26, 27. When in the "off" condition each of the filters is arranged to transmit all wavelengths fed along the respective branch circuits substantially completely so that there is no significant loss of traffic wavelengths (for example 1200 to 1570nm) or of the OTDR wavelengths. However each of the filters is arranged to be individually switched to an "on" condition by a respective interrogation signal in which it is substantially wholly reflective of the OTDR wavelengths, although still transmissive of all other wavelengths. Accordingly OTDR signals that are returned to the OTDR equipment are indicative of the state of the terminal branch containing the consumer equipment having the interrogated filter. If each of the filters is interrogated in turn it is a simple matter to isolate any faulty branch or branches, in dependence on the nature of the returned OTDR signals. In use, when a reflector at distance d is "on", measurements are made on the mirror or "virtual" image of the path from terminal to the OTDR, ie. at distance d-2d, rather than in the "real" image at distance 0-d, this facilitating differentiating between returned signals. The system has the advantage that, as the filters are located on customers' premises, power is readily available to provide for interrogation of the filters, thereby avoiding the need for separate power supplies to remote positions which may be necessary for such a purpose in cases where the filters were to be located in other parts of the branch network. Figure 3 represents a modification of the network above described. In this case each of the filters 25, 26, 27 is disposed in a spur line 35, 36, 37 so that it does not pass traffic to the consumers equipment, but is similarly arranged to reflect back towards the OTDR equipment OTDR signals when appropriately interrogated. Traffic signals can either be transmitted or attenuated, so that they are not reflected.

Claims: Fault location apparatus for use in an optical signal transmission network of a type which includes a main optical fibre (8) having a plurality of branch fibres (11,12,13,14), in which each final branch fibre (11,12) of a chain of branches ends in a terminal unit (15,16,17), said fault location apparatus comprising: optical time domain reflectometry (OTDR) equipment (1,2,3) connected to the optical transmission network (8,11,12,12,14) adapted to transmit an output signal through the main fibre (8) and the branch fibres (11,12,13,14); and characterised in that there is a filter (25,26,27) incorporated in each final branch fibre (11,13,14) which is adapted to reflect towards the OTDR equipment a wavelength or wavelengths unique to that branch in the OTDR band, but not traffic wavelengths, and wherein said OTDR (1,2,3) equipment is operated to make measurement on a "virtual" or "mirror" image of the path from the terminal unit (15,16,17) and has means for distinguishing the signals returned from individual final branch fibres (11, 13, 14). Fault location apparatus, according to Claim 1, in which each filter (25,26,27) is disposed in the traffic line (11,13,14) and is substantially transmissive to traffic wavelengths. Fault location apparatus, according to Claim 1, in which each filter (25,26,27) is disposed in a spur line connected to the traffic line and is such as to transmit or attenuate traffic wavelengths. Fault location apparatus, according to Claim 1, 2, or 3, wherein each filter (25,26,27) is switchable between a first condition in which it transmits or attenuates wavelengths in the OTDR band, and a second condition in which it reflects wavelengths in the OTDR band, and means are provided for switching said filters (25,26,27) individually into said second condition whilst maintaining the remaining filters (25,26,27) in the first condition. Fault location apparatus, according to Claim 4, wherein means are provided for individually interrogating the filters (25,26,27) for switching them in turn from the first to the second condition. Fault location apparatus, according to any preceding claim, wherein each said filter (25,26,27) is located in or adjacent to the respective terminal unit.

Inventors: SHIPLEY SIMON PAUL, SHIPLEY, SIMON PAUL
Applicants: MARCONI COMM LTD, MARCONI COMMUNICATIONS LIMITED
Application year: 1994
Main classification: H04B10
IPC codes: H04B10, G01M11